1. Field of the Invention
The present invention relates to an organic light emitting element applicable to a flat panel display and to a light source for illumination. In particular, the invention relates to a high-efficiency organic light emitting element capable of reducing the power consumption of a display.
2. Background of the Related Art
In general, a color-conversion-type color display using an organic light emitting element employs an organic light emitting element (hereinafter, referred to as a blue to blue-green organic light emitting element), transmits blue light using a blue color filter for a blue pixel, and converts the wavelength of EL light using a color conversion layer to obtain red light for a red pixel.
For green, the color-conversion-type color display transmits a green component in the EL light using a color filter, or uses a color conversion layer for emitting green light to obtain green light.
As the organic light emitting element, the blue to blue-green organic light emitting element is used for each of the red, green, and blue pixels. Therefore, in the color-conversion-type display, the emission efficiency of the blue to blue-green organic light emitting element has a great influence on the performance of the display, such as power consumption.
As a means for improving the emission efficiency of the organic light emitting element, a method has been proposed in which a material obtained by doping a host material with a guest material is used to form a light emitting layer, for example, see Japanese Patent Application Laid-Open (JP-A) No. S63-26469. This method transfers energy from the host material excited by the recombination of holes and electrons to the guest material to emit fluorescence from the guest material. Since the guest material is doped at a low concentration of several percent, concentration quenching by the reaction between the guest materials does not occur and it is possible to emit light from the guest material substantially in an ideal stage. Factors determining the emission efficiency of this method include, for example, the probability that singlet excitons will be generated by recombination on the host material, the transfer efficiency of energy to the guest material, and the fluorescence quantum yield of the guest material.
As a method of further improving efficiency, a method of using phosphorescence has been proposed, for example, see M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, Applied Physics Letters, volume 75, issue 1, p.4 (1999). Since phosphorescence is the emission of light in a triplet state, the probability that the excitons will be generated is high and it is possible to greatly improve emission efficiency.
In addition, there is an attempt to develop a technique for improving the efficiency and reliability of a fluorescent organic light emitting element using the above-mentioned doping technique. For example, a technique has been proposed which forms an organic EL layer with a host material including a first dopant capable of accepting electron/hole binding energy and a second dopant capable of trapping holes, for example, see JP-A No. 2002-38140.
As another example, two kinds of organic light emitting elements have been disclosed in which a light emitting layer includes a host material and first and second dopants satisfying the following relationships, for example, see JP-A No. 2002-38140, and one of the two kinds of organic light emitting elements satisfies the following conditions:EV0>EV1 and EV0>EV2  (A)EC0≧EC2  (B)Eg0>Eg1,Eg2,  (C)wherein EV0, EV1, and EV2 indicate the valence electron levels of the host material, the first dopant, and the second dopant, respectively, EC0 and EC2 indicate the conduction levels of the host material and second dopant, respectively, and Eg0, Eg1, and Eg2 indicate the energy gaps of a light emitting layer material, the first dopant, and the second dopant, respectively.
The other of the two kinds of organic light emitting element satisfies the following conditions:EV0>EV1 and EV0>EV2  (A′)EC0≧EC1,EC2,  (B′)wherein EV0, EV1, and EV2 indicate the valence electron levels of the host material, the first dopant, and the second dopant, respectively, and EC0, EC1, and EC2 indicate the conduction levels of the host material, the first dopant, and the second dopant, respectively.
However, in the phosphorescence method disclosed in M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and S. R. Forrest, Applied Physics Letters, volume 75, issue 1, p.4 (1999), when blue to blue-green light required for the color conversion type is emitted, a host material having triplet energy suitable to emit blue to blue-green light, that is, having a large band gap, is needed. However, at present, the choice of the host material is limited and a substantial host material capable of obtaining both high emission efficiency and driving stability has not been proposed yet. In addition, for driving stability, since the duration of phosphorescence is longer than that of fluorescence, a light emitting material in an excited state is likely to react with another material to be quenched. As a result, the lifespan of the material is reduced. Therefore, a breakthrough is needed in order to put the host material to practical use.
In the example of the technique disclosed in JP-A No. 2002-38140, emission efficiency and emission lifetime are improved, as compared to the example in which only one kind of dopant is used. However, JP-A No. 2002-38140 does not disclose an example in which the technique is applied to a blue to blue-green organic light emitting element. In addition, the organic light emitting element having a structure in which two kinds of dopants both can emit light further includes a carrier transport dopant to improve the emission efficiency and emission lifetime, but has a problem in that the driving voltage is substantially high. This is because the two kinds of dopants both have the property of capturing electrons.
The blue to blue-green organic light emitting element disclosed in JP-A No. 2004-171828 is capable of improving brightness efficiency and increasing driving lifetime, without increasing the driving voltage, as can be seen from the comparison between examples and comparative examples. However, the brightness efficiency and the driving lifespan are performance levels that can also be obtained by the use of a single dopant. Therefore, a noticeable effect is not obtained.
The invention has been made in view of the above-mentioned problems, and an object of the invention is to provide a blue to blue-green organic light emitting element having high brightness, high efficiency, and a long lifespan.